This invention relates to a method that can measure Fe coating weight of an Fe-coated stainless steel sheet at the same time when Fe-coating is carried out.
Stainless steel sheets have superior corrosion resistance and heat resistance. Application of hot dip coating of Al or the like to stainless steel sheets can bring about a more improvement in corrosion resistance and heat resistance. This hot dip coating is carried out by the same coating method as in the hot dip coating on low-carbon steel sheets. This method commonly includes a method in which oily deposits such as rolling mill lubricants and rust preventives having adhered to the surface of a base steel sheet are first removed in a non-oxidizing furnace or degreasing device of a continuous hot dip coating line and then the Fe oxide on the surface is reduced in a high-temperature H.sub.2 --N.sub.2 atmospheric gas, followed by hot dip coating of Al or the like. When, however, the hot dip coating is applied to stainless steel sheets by this method, Cr contained in stainless steel sheets may thermally diffuse at the time of the reduction to concentrate toward the surface, so that it preferentially undergoes oxidation to cause damage of wettability to molten metals, resulting in occurrence of surface defects such as non-coating.
Accordingly, when hot dip coating is applied to stainless steel sheets, Fe is previously coated in a continuous electroplating line in a coating weight of 0.05 to 5 g/m.sup.2 to make their surfaces have the same properties as those of low-carbon steel sheets, followed by hot dip coating in a continuous hot dip coating line. This Fe pre-coating is applied to improve the wettability of stainless steel sheets to molten metals. Hence, in order to produce hot dip coated stainless steel sheets at a low cost, it is preferable to make the Fe coating weight smallest so long as no surface defects may occur.
For measuring this Fe coating weight, no method for accurately measuring it at the same time when Fe-coating is carried out in a continuous electroplating line has been hitherto available. Accordingly, the Fe coating weight has been measured by a gravimetric analysis in which a sample is taken and its Fe coating layer is dissolved with an aqueous nitric acid solution so that the coating weight is calculated from a difference in weight before and after the dissolution.
In this gravimetric analysis, however, the sampling has been limited to tops and ends of Fe coated stainless steel sheet coils and no coating weight can be ascertained on the lengthwise extent of Fe coated stainless steel sheet coils. Hence, it has been common to apply Fe-coating in a little larger coating weight to prevent shortage of coating weight. This has brought about the problem that Fe-coating results in a high cost. There has been another problem that, because of a long time taken after the sampling until measurements come out, no immediate adjustment can be made even if the coating weight has been found excessive or insufficient. There has been still another problem that this gravimetric analysis can not be used when a continuous electroplating line is set on the inlet side of a continuous hot dip coating line so that Fe pre-coating and hot dip coating can be simultaneously carried out in order to improve productivity, because samples can not be taken for only Fe pre-coating.
As measurement of coating weight not relying on the gravimetric analysis, X-ray fluorescent analysis is known in the art. This is a method in which the fluorescent X-ray intensity ascribable to an element constituting a base stainless steel sheet or the fluorescent X-ray intensity ascribable to an element mainly constituting a coating layer is measured and the measurements are substituted in a fluorescent X-ray intensity equation to calculate the coating weight. Hence, this requires no sampling, and may be considered feasible for continuous measurement of the coating weight.
This method, however, is not feasible for measurement of coating weight since, when both a base stainless steel sheet and a coating layer contain Fe as in the case of Fe-coated stainless steel sheets, the fluorescent X-ray intensities ascribable to Fe contained in the both can not be separated. Thus, this method can only be applied to instances in which metals different from the components of base sheets are coated.